The present invention relates to a method for controlling the solidification process of a casting melt, by determining the intrinsic crystallisation ability of a melt and making those corrections that are necessary.
When producing cast iron of all conceivable types, it is very important to check the number of graphite crystals formed per unit of volume. In white cast iron no graphite crystals are formed; mottled cast iron has a few graphite crystals; grey cast iron and cast iron with compact graphite have a moderate number of graphite crystals; and nodular iron normally has a large number of graphite crystals.
Consequently, a nucleation stimulant is introduced into the melt shortly before the casting process, in order to stimulate the formation of a desired number of graphite crystals. A large number of nucleation stimulants are commercially available, the majority of these stimulants being based on ferrosilicon (FeSi) or silicon carbide. Many of these stimulants contain so-called inoculating agents and also certain additive elements, such as calcium, strontium or zirconium, with the intention of amplifying the effect of the stimulants.
The effect produced by the inoculating agents is very shortlived, however, and these agents must therefore be added to the melt at a late stage of the casting process, often in the casting jets or even in the actual casting mould itself. It is obvious that the effect of such additives is difficult to monitor and control in a manner to achieve optimal results, since the inoculating effect achieved will vary from melt to melt, and therewith from product to product.
The mechanism through which nucleation of graphite crystals takes place in the presence of FeSi-particles (the substance is most normally added in the form of coarse granules having a size of 1-10 mm) is well described in the literature (see for instance Ch Wang and Fredrikson; 48th International Casting Congress in Varna, Bulgaria, 1981-10-4-7,255).
The carbon equivalent (i.e. C.E.=% C+% Si/4) will increase sufficiently in the diffusion zone that occurs when an FeSi-particle is dissolved in the melt for a graphite crystal to graphitize in the melt, provided that this small graphite crystal survives until its normal growth temperature has been reached (i.e. generally at a temperature &lt;1155.degree. C.). Under such circumstances, the graphite crystal is able to develop into a flaky graphite crystal or a graphite nodule, depending upon the chemical environment prevailing in the iron melt. Whereas Wang and Fredrikson state that the formation of graphite crystals takes place through a homogenous nucleation process, several other authors, for instance Jacobs et al, Metals Technology, Mar. 1976, page 98 (page 102) state the opinion that the formation of graphite crystals is a heterogenous nucleation process. These authors have namely found in graphite crystals primary crystallisation nuclei which consist of complex oxides of such elements as calcium, magnesium and aluminium of the spinel type, which are thermodynamically stable and well dispersed in the melt. The present invention is based on the significance of these so-called primary nuclei.
The concentration of such primary nuclei in the base melts used in present day casting technology varies considerably, partly due to the starting material used; this starting material ranges from sponge iron, material recycled from the foundry concerned, to steel scrap and more or less well defined scrap purchased on the market.
The melting method used also plays an important part. Furnaces operate in accordance with different principles (for instance gas-fired or oil-fired cupola furnaces, light-arc furnaces and induction furnaces), which heat the base iron to different temperatures during the melting process. Furthermore, the furnace linings influence the sulphide, oxysulphide and oxide particles in the molten material. Consequently, the concentration of primary nuclei in the base melt will vary very widely, not solely from the one production line to the other, but also from batch to batch in one and the same production line.
It is known that many patent specifications disclose valuable information concerning the properties of a melt. SE-B-350 606 in particular teaches a method in which a sample of the melt is taken in a sampling vessel when casting aluminium and the temperature changes that take place in time as the melt solidifies are recorded with the aid of a thermoelement placed in the melt. These records are then used to anticipate crystallisation conditions on the basis of undercooling values, the slope of-different parts of the curve, and constant temperatures during the eutectic reaction. SE-B-444 817 teaches a method by means of which information concerning the properties of the melt can be obtained such as to be able to determine whether the melt will solidify as flaky graphite iron, as a compact graphitic iron or as nodular iron. This information is obtained with the aid of two thermoelements, one of which is placed in the melt in the centre of a sampling vessel and the other is placed in the melt in the proximity of the wall of said vessel.